You can purchase and download audio books from iTunes. They play on the iPod app.
If you have audio books that you did not get form the iTunes Store, and if they are not DRM, then you can drag them to iTunes on the computer. You may have to select them and edit their info to make them show up in the Books directory. Right click on the song, choose get info, go to the Options tab and select Audiobooks as the media kind.
Once there you can set your iPad sync preferences to sync all, or selected audio books. Just hook up your iPad, select it in iTunes, go to the books tab and make your choices.
Sync the iPad.
Your audio books should show up in the iPad app under, well, audiobooks.